Page:A Treatise on Geology, volume 1.djvu/179

 CHAP. VI. as flagstone. In colour these rocks are white, brown, grey, greenish, yellow, or red. There is almost every possible gradation between the sandstones and argillaceous deposits; which latter are frequently much laminated, and are then called plate, or bass; less remarkable lamination causes shale; deficiency of lamination belongs to some varieties, associated with coal, called clunch, bind, and other local names: most of them are more or less bituminous; colour blackish, greyish, bluish, yellowish. The limestones are compact or oolitic, or granularly crystallised; mostly pure carbonate of lime (except the granular sorts, which usually contain magnesia), white (rarely yellowish), grey, blue, black, red, or mottled. Some beds contain quartz pebbles. Nearly all are of marine origin, but some exceptions occur.

Chert nodules and beds, of white, black, yellow, or red colour, lie in the limestone, like lumps and layers of flint in chalk; and require similar suppositions to explain their occurrence. Some considerable beds of chert occur in the north of England (Swaledale), and many sandstones are of a cherty nature (Harrogate).

Ironstone (a carbonate of iron) often accompanies the thick dark plates and shales, in rows or layers of nodules (see Diag. No. 21, p. 61.), aggregated round shells (unio), fern branches, &c. Coal lies always in beds. Its quality varies from nearly pure carbon to a consumable mixture of carbon, hydrogen, oxygen, and azote; and it is often mixed with layers of woody fibre, like charcoal, and laminæ of earthy matter.

Structure.—Through out all this mass of varied deposits in the carboniferous system, the most decided proofs of aqueous deposits constantly present themselves. Lamination belongs, but not equally, to every one of the six constituent members; being often conspicuous in sandstones (flagstones), almost always so in argillaceous rocks and coal; frequent in black limestones, but rare in ironstone. Real beds occur in all these rocks; but in the argillaceous plates and shales they are often indiscernible;